Landing pads are well known in the art. They are used in a semiconductor device to provide electrical contact from one conductive layer, typically a metal layer to a conductive polysilicon layer through an insulating layer. Typically, conductive polysilicon is used in the formation of a logic or memory circuit, and the metal layer is used to carry signal and/or power to or from the circuit.
Referring to U.S. Pat. No. 6,329,685, whose disclosure is incorporated herein in its entirety, there is shown a non-volatile memory cell with a control gate made of conductive polysilicon in the shaped of a spacer. See FIG. 2H-4 thereof. This is also shown as FIG. 1 hereof. The memory cell 10 comprises a semiconductor substrate 11, of a first conductivity type, typically P type. The cell 10 comprises a first insulating layer 12 on the substrate 11. A floating gate 14, having a tip that permits Fowler-Nordheim tunneling of charges from the floating gate 14 to the control gate 40, is also made from conductive polysilicon and is formed on the first insulating layer 12. The floating gate 14 is also capacitively coupled to a region 30 in the substrate 11. A source contact 34, also typically made from a conductive polysilicon, electrically connects to the region 30 in the substrate 11. The source contact 34 is also insulated from the floating gate 14 by a second insulating layer 26. The structure 20 comprising the floating gate(s) 14, the first insulating layer 12, the source contact 34 and the second insulating layer 26 is generally rectangularly shaped, and has a substantially planar surface against which the control gate 40 in the shape of a spacer is formed. The spacer shaped control gate 40 is made of conductive polysilicon. As is well known, to form a spacer, polysilicon is conformally deposited on the structure 20. The polysilicon is then subject to an anisotropic etch which results in the spacer shape. The spacer shaped control gate 40 can be made conductive by, for example, ion implantation, either before the polysilicon is etched, or after the polysiclion is etched, i.e. after it is shaped into a spacer shape.
Referring to FIG. 2 there is shown a top view of the structure shown in FIG. 1. Generally, the structures 20 are fabricated as parallel strips, parallel to one another, with the control gate spacers 40, immediately adjacent to the structure 20, and therefore also parallel to one another.
The structure shown in FIG. 1 is further fabricated to form additional regions in the substrate 11, each of which is spaced apart from an associated region 30, and is between a pair of adjacent spacer control gates 40. Thereafter, insulating material (not shown), such as BPSG or any other form of glass or oxide material is deposited. Finally, landing pads are formed through the insulating material to contact the spacer shaped control gate 40. A landing pad, such as 50 shown in FIG. 2 is a hole or via, made in the insulating material that covers the structure shown in FIG. 1, so that a metal contact to the conductive polysilicon control gate 40 can be made. Although the formation of landing pads is well known, the formation of a landing pad to a spacer shaped conductive material, and in particular one that is used as a control gate creates a special problem.
Referring to FIG. 2, there is shown a landing pad 50 that is located to make electrical contact with the control gate 40. The formation of a landing pad 50 to a memory array in which rows of control gates 40 are formed parallel and spaced apart from one another means that the landing pad 50 must be accurately positioned in the X direction. If there is any significant deviation in the X direction, the landing pad 50 might make contact with the “wrong” row of control gate, i.e. 40c instead of 40b. Alternatively, the landing pad 50 might contact the source contact 34. Furthermore, the problem of making a landing pad 50 contacting a spacer conductor 40 is further exacerbated by the shape of the conductor 40. Thus, even if the landing pad 50 is positioned within the range of tolerance, i.e. it does not contact the control gate 40c nor make contact with the source contact 34, because the spacer control gate 40 is curvilinearly shaped, the depth of the landing pad 50 required to make contact may differ significantly from one landing pad to another, leading to potential poor electrical contact.
Hence there is a need to develop a landing pad which can be used to make electrical contact with a spacer shaped conductive member.